Had he done it two weeks ago, he could've withdrawn and probably even got a policy deal from Arnold (some concessions on illegals or even on the budget) to hang his hat on -- and been a rising star on the state scene. Now, he has forfeited all that to Issa, who will get first dibs on anything good to open up in the state.
The greatest danger for McClintock was to wait until Arnold starts seeing that McClintock's presence in the race is doing him no significant harm, the point where all of McClintock's leverage disappears: And McClintock's leverage has now disappeared -- if he leaves the race, he leaves with nothing to show for it. If he stays in, he has no impact. And now people are wondering if his staying might actually increase the "Yes" vote on question 1 (it probably does, though again only to some modest effect). Now he is in danger of becoming so irrelevant that people backing Arnold will urge him to stay in to give that little boost to question 1.
Question 1 is certain pass -- that will become more and more apparent the closer we get to 10/7. And so, the closer we come to the election, the more irrelevant McClintock becomes. This has been a great strategic error on his part, one that he should have seen from miles away. Timing was everything for him, and he ignored that basic fact.
In the last week of the campaign, it will get so bad for him that people everywhere will be saying "McClintock, who?" And much of his support will drain away of its own account, without anyone doing anything to help it along. He missed his moment, and used extremely poor strategy. No wonder he has no great accomplishments to show for all of his years in the legislature.